1. Field of the Invention
The invention relates to a method and apparatus for estimating well bore formation temperature by measuring the temperatures of the drilling mud within the drill string and in the annulus of the borehole and, more particularly, the determination of formation temperature and mud thermal conductivity by establishing a well bore thermal model.
2. History of the Prior Art
It has long been recognized in the oil industry that the collection of downhole data during drilling is of extreme value. Such information as to downhole environmental parameters aids in locating specific earth formations. Moreover, it improves the efficiency of the drilling operation by providing a "feed back" as to in situ temperature, volume, viscosity, resistivity and pressure conditions. This information often mandates modifications in the drilling operation. For example, the drilling mud consistency is directly affected by the cuttings from the drill bit. Thus, efficient drilling requires proper selection and maintenance of drilling fluid characteristics and the viscosity and flow rate of the mud is often varied in response to downhole conditions. The value of mud thermal conductivity is another parameter which is useful in mud selection and in controlling the mud flow behavior at the bit.
Certain downhole data is both useful from an efficiency point and may serve as a warning to prevent dangerous situations from arising. Downhole formation temperature is one such parameter. For example, an ever present danger in the drilling of a borehole is encountering earth formations which contain high pressure fluids such as gas or hot salt water. When a gas zone is penetrated, the high pressure fluids from the formation enter the borehole and displace the drilling mud back up the borehole toward the drilling rig at the surface. If the penetration of a high pressure formation and the ensuing direct intrusion of high pressure fluids back into the borehole is not detected quickly, for example, by a sudden temperature change, and controlled, it can result in the complete displacement of the drilling mud back up the borehole. This even results in the expulsion of the high pressure fluids out of the top of the borehole and is called a "blow out". A blow out can result in great injury to both property and life due to the high combustibility of the gas and other fluids egressing from a well bore and the violence with which they exit.
On the other hand, it is possible that, during drilling, a borehole may enter a formation which is highly porous and create a tendency for all of the drilling mud to flow freely from the borehole into the porous formation. This event is termed "lost circulation" and can result in the substantial loss of drilling fluids if the lost circulation is not detected very rapidly and preventive measures taken. Preventive action is needed upon the detection of an earth formation from whence such an event might be likely. It is thus desirable to detect such a formation as rapidly as possible in order to take the requisite remedial action to control mud flows. The accurate and rapid measurement of the formation temperatures along the borehole would enhance the aforesaid detection efforts as well as provide much needed data for locating specific earth formations.
The measurement of earth formation temperature in a measuring while drilling mode has been proven to be a difficult and costly endeavor. One technical problem of concern is the number of variables which are involved. One such variable is the thermal conductivity of the drilling fluid which changes constantly. The value of thermal conductivity, itself, is a useful parameter for enhancing the selection of drilling fluids as well as providing a vital link in the determination of formation temperature. With such data and proper extrapolations therefrom, abnormally pressured formations can be predicted and anticipated. Abnormally high pressured formations will induce a higher than normal formation temperature gradient. Such information would obviously be extremely valuable.
It is well known to "survey" the temperature of the drilling mud in a borehole to detect a "blow-out", "wash-out", a locked bit and indirectly the porosity and fluid content of the formation. A substantial increase in the temperature of mud can indicate the presence of hot salt water. A sudden decrease in the temperature of the mud can indicate the presence of the cooling effect of expanding gas. Such data does not, however, directly indicate the temperature of the earth formation around the borehole, because it has only been possible to make such measurements of the mud itself and not the formation in a measuring while drilling mode. For example, U.S. Pat. No. 3,327,527 (Arps) discloses a system for measuring the thermal gradients in drilling mud down in the borehole where the event in issue actually occurs. Temperatures of the mud are taken inside the drill collar and in the annulus of the borehole. This information is telemetered back to the well head and used to determine the presence of heat gain and heat loss to indicate both hazardous conditions and, indirectly, the character and drillability of the formation. It is well known that without further information, such readings do not indicate the actual temperature of the formation with any accuracy.
In actual practice the process is not as obvious as disclosed by Arps. Only recently, 1984, have effective downhole temperature measurements been made by measurement-while-drilling equipment. The thermal variations due to bit conditions, formation inconsistencies, bit motion which causes erratic fluid motion, and very high flow rates in modern day drilling practices, yield several conditions which make the teachings of Arp unuseful.
First, the high flow rates make the temperature differential along the annulus very small, and indicates that the temperature difference expected over the bottom 100 foot interval is less than 1.degree.. This means that the estimation of the formation thermal conductivity taught by Arps is dependent upon an extremely small temperature difference. These thermal variations are masked by the thermal variations caused by the bit dynamics. The varying weight on the bit and the microscopic variations in formation hardness creates variation in drilling energies which create heat input to the drilling mud. These variations which can easily result in a few degrees of temperature variation, easily mask the thermal conductivity changes of the formation. These variations must be overcome by discrimination or compensation. Methods which accomplish this are not obvious, since these problems were not obvious to Arps.
The problem can be approached from an unique approach; that is from measuring temperature variations across the drill pipe when the drilling is stopped every 30 feet to add drill pipe. Other prior art approaches address that goal.
A prior art approach to determining earth formation temperature is the measurement of the ambient temperature at the drill bit with the drilling operation interrupted. For example, U.S. Pat. No. 3,455,158 (Richter, et al) sets forth apparatus for logging the temperature of a drill bit, drilling mud and earth formation. A temperature transducer is mounted directly in the drill bit for this purpose. The bit engages the earth formation and thus may reflect the temperature thereof once the effect of drilling mud is eliminated. Unfortunately, the time necessary for the downhole thermal conditions to sufficiently stabilize for an accurate "ambient" reading is several hours. This factor alone prevents the system from being widely used to determine earth formation temperature because it is not cost effective to repeatedly interrupt drilling for such long periods of time.
It is recognized that the drilling mud itself does reflect certain downhole thermal and geophysical conditions. Either the downhole temperature or surface temperature of the returning mud flow can indicate abrupt changes of temperature of the magnitude encountered prior to penetration of high pressure formations. Moreover, the cuttings returning in the mud are a good indication of the formation density. One such technique is set forth in U.S. Pat. No. 3,701,388 wherein the drilling fluid temperature is measured at the surface to detect such zones and prevent lost circulation and blowout of the well. Such temperature measurements are made without regard to the thermal conductivity of the fluids and materials involved. For this reason, a determination of the actual formation temperature is not realistically offered or even suggested by the telemetered data. It is the quantitative change which alerts the operator, not qualitative information such as actual formation temperature.
It would be an advantage to accurately estimate earth formation temperature by downhole sensors without having to wait hours for the downhole ambient temperature to stabilize. The present invention overcomes the aforementioned prior art problems by providing a well bore thermal model incorporating the heat generation, heat transfer and heat storage parameters of the drill pipe, mud and bit. Borehole annulus and drill pipe temperature measurements are taken by downhole sensors over a relatively short time frame during which temperature differentials maximize. In this manner, drilling mud thermal conductivity can be calculated and earth formation measurements estimated in a single time span prior to thermal stabilization.